Non-invasive analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis, for example, for quality control purposes.
Optical coherence tomography (OCT), is a technology for non-invasive imaging and analysis. OCT typically uses a broadband optical source, such as a super-luminescent diode (SLD), to probe and analyze or image a target. It does so by applying probe radiation from the optical source to the target and interferometrically combining back-scattered probe radiation from the target with reference radiation also derived from the optical source.
The typical OCT optical output beam has a broad bandwidth and short coherence length. The OCT technique involves splitting the output beam into probe and reference beams, typically by means of a beam-splitter, such as a pellicle, a beam-splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). Light or radiation is scattered by the target, some of which is back-scattered to form a back-scattered probe beam, herein referred to as signal radiation.
The reference beam is typically reflected back to the beam-splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam-splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal.
Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. In a conventional time domain OCT system, a measurement of the scattering values at various depths can be determined by varying the magnitude of the reference path length, typically by moving the reference mirror. In this manner the scattering value as a function of depth can be determined, i.e. the target can be scanned.
There are various techniques for varying the magnitude of the reference path length. Because electro-mechanical voice coil actuators can have considerable scanning range, however, there are problems with maintaining the stability or pointing accuracy of the mirror. Fiber based systems use fiber stretchers, however, fiber stretchers have speed limitations and have size and polarization issues. Rotating diffraction gratings can run at higher speeds, however, are alignment sensitive and are too bulky.
Piezo devices can achieve high speed scanning and can have high pointing accuracy, however to achieve a large scanning range requires expensive controls systems and have limited high speed capability.
A scanning method that effectively amplifies the scan range of a piezo device is described in the patents numbered U.S. Pat. Nos. 7,526,329 and 7,751,862. The method taught in these patents uses multiple reference signals with increasing scan range and correspondingly increasing frequency interference signals.
While the multiple reference scanning method can achieve a relatively large scan range at high speed with good pointing stability, there are applications, such as ophthalmic applications, that require imaging or measurements can span significantly larger distances than can be spanned even by amplified piezo scans. In the ophthalmic application, information spanning the full axial length of an eye (of the order of 28 mm) is required.
There is therefore an unmet need for a method, apparatus and system that can achieve large scan range of up to approximately 30 mm at high speed with good pointing stability.